1. Field of the Invention
This invention relates to a process for producing toluene diamine from an impure dinitrotoluene reactant.
2. Description of the Prior Art
U.S. Pat. No. 2,131,734 is typical of the early processes for the liquid phase hydrogenation of dinitrotoluene to toluene diamine. Typically, the hydrogenation was carried out in the presence of nickel supported on a silica substrate and in the presence of a small amount of an alkaline substance such as caustic soda. Typical amounts of catalysts were 0.5-2% catalyst metal by weight of the dinitrotoluene. Temperatures were from 80.degree. to 140.degree. C. and pressures were from 200 to 500 pounds per square inch.
U.S. Pat. No. 2,976,320 represented an improvement over the early liquid phase hydrogenation processes of dinitrotoluene. In that process, the temperature of reaction was reduced to less than 100.degree. C. to prevent hazards due to the instability of the dinitrotoluene. Low temperature hydrogenation was accomplished by first reducing the nitrocresol content in the feed and then using a palladium or platinum catalyst to effect hydrogenation. It was reported that nitrophenols or nitrocresols, which are normally present in commercial dinitrotoluenes, are catalyst poisons and decomposition accelerators, and that by limiting the concentration to less than 500 ppm, and preferably 200 ppm, a dinitrotoluene could be hydrogenated safely at low temperatures. One of the typical ways of removing the nitrocresols and nitrophenols was by washing the crude dinitro product, obtained by separation from the spent nitration acid, with dilute aqueous alkali (e.g., sodium carbonate solution). The product then was water washed.
U.S. Pat. No. 3,093,685 discloses a liquid phase process for reducing dinitrotoluene to toluene diamine with the improvement residing in carrying out the reduction in the presence of water of reaction with no other solvent or diluent being added.
U.S. Pat. No. 3,356,728 differs from the patents previously cited in that it relates to a continuous process for reducing dinitrotoluene to toluene diamine. It was disclosed that various operating problems such as poor catalyst life and operating hazards due to the instability of the dinitrotoluene could be minimized by using the continuous process wherein the catalyst was present in about 2 to 25% by weight of the slurry with extremely low concentrations of dinitrotoluene as the reactant. The level of nitro compound was maintained at 0.15 pound equivalents nitro groups per pound of catalyst. Temperatures of 100.degree.-140.degree. C. and pressures of 100-2000 psig, preferably 200-500 psig hydrogen were used. Raney nickel catalyst was used in the hydrogenation procedure.
U.S. Pat. No. 3,546,296 relates to another continuous process for producing toluene diamine by the reduction of dinitrotoluene utilizing specific reactor equipment. Again, the concentration of dinitrotoluene in terms of the catalyst is extremely low so as to reduce operating hazard.